1. Field of the Invention
The present invention relates to an optical disk device for recording information on an optical disk such as a compact disc (CD) or a CD-ROM or for reproducing recorded information from the optical disk, and particularly to an optical pickup actuator circuit for forming an optical pickup for recording/reproducing information.
2. Description of the Related Art
An actuator circuit including a coil and a magnet for focusing or tracking control is generally used in an optical pickup of an optical disk device.
As an example of the optical disk device having such an optical pickup actuator circuit, there has been proposed a biaxial actuator aimed at performing proper signal reproduction (Japanese Patent Application Laid-Open No. 9-237428).
The biaxial actuator is structured such that an objective lens holder of an optical pickup holds objective lenses respectively corresponding to a plurality of different-form optical disks at different angular positions on one and the same circumference with respect to a support shaft. The lens holder is rotated around the support shaft by interaction between a tracking coil and a tracking magnet so that one of the objective lenses is inserted into an optical path.
Hence, an image of a light beam from a light source is formed correctly on a signal recording surface of an optical disk through the objective lens. Then, a return beam from the signal recording surface of the optical disk correctly enters a photo-detector. In this manner, signal reproduction optimal to each of the different-form optical disks is performed.
There is also known an optical disk device aimed at solving a problem that the device is damaged mechanically by the malfunction of a focusing servo operation portion (Japanese Patent Application Laid-Open No. 4-23235).
The optical disk device is structured such that, when abnormality occurs in rotation of an optical disk, the tracking operation of the focusing servo is stopped to prevent a laser beam from being focused onto an optical disk surface before the laser output is reduced or stopped to thereby protect the optical disk optically and mechanically.
Incidentally, in any one of the related-art optical disk devices, the focusing and tracking coils are disposed on the lens holder for holding the objective lenses. It is therefore impossible to avoid such a problem that the focusing and tracking coils may be burned out by an abnormal voltage so that the disk device becomes disabled.
That is, in the case of a focusing coil, as shown in FIG. 6, the focusing coil 7 has one input end 7a connected to a positive pole terminal 15a of a focusing operation portion 15, and the other input end 7b connected to a negative pole terminal 15b of the focusing operation portion 15.
The focusing operation portion 15 is connected to a disk drive controller 11 through a focusing and tracking servo circuit 12.
The disk drive controller 11 sends a control signal to the focusing and tracking servo circuit 12 in accordance with a command signal at the time of reproduction. The focusing and tracking servo circuit applies, for example, 0.7 V as an operating voltage to the focusing operation portion 15.
Accordingly, the signal voltage is applied across the opposite ends of the focusing coil 7, so that an operation signal of 0.7 V is led from one input end 7a to the other input end 7b. 
Hence, an objective lens is operated so as to move vertically in the optical path, and an image of a light beam from the light source is formed on a desired track of a signal recording surface of the optical disk through the objective lens.
In such an optical pickup actuator circuit, however, an abnormal voltage maybe input to the focusing operation portion 15 at the time of the focusing operation because of short-circuit due to bad soldering of a wiring substrate or other causes.
If such an abnormal voltage is applied across the opposite ends of the focusing coil, the focusing coil may be broken due to burning out to make the objective lens inoperative. Indeed, there was such a disadvantage that recorded information could not be reproduced.
Such breaking of the focusing coil could be found only in a test performed after production. Hence, there was a disadvantage that the efficiency of producing the optical disk devices was lowered greatly.